Clarify some query snippets

See: firebase/snippets-web#186

Author: samtstern

firestore/app/src/main/java/com/google/example/firestore/DocSnippets.java

@@ -1019,13 +1019,13 @@ public void simpleQueries() {
         // [END simple_query_capital]
 
         // [START example_filters]
-        citiesRef.whereEqualTo("state", "CA");
-        citiesRef.whereLessThan("population", 100000);
-        citiesRef.whereGreaterThanOrEqualTo("name", "San Francisco");
+        Query stateQuery = citiesRef.whereEqualTo("state", "CA");
+        Query populationQuery = citiesRef.whereLessThan("population", 100000);
+        Query nameQuery = citiesRef.whereGreaterThanOrEqualTo("name", "San Francisco");
         // [END example_filters]
 
         // [START simple_query_not_equal]
-        citiesRef.whereNotEqualTo("capital", false);
+        Query notCapitalQuery = citiesRef.whereNotEqualTo("capital", false);
         // [END simple_query_not_equal]
     }